Embodiments of the invention relate generally to power converters and, more particularly, to a DC-DC power converter that provides galvanic isolation and an adaptive conversion ratio.
Switch mode DC-DC converters are widely used for converting a given input electrical power to a desired output electrical power, with such DC-DC converters being able to function as boost-type converters (converting an input voltage to a higher output voltage), buck-type converters (converting an input voltage to a lower output voltage), or converters capable of both boosting or bucking the voltage. In addition, they can be classified as unidirectional or bi-directional converters based on their ability to flow power. In switch mode DC-DC converters, the input power is provided from a source to the converter through input terminals on an input side, converted by the converter into the desired output power and then output through output terminals on an output side provided to a load. The converter comprises a switching arrangement and power transformer for transferring the electrical power from the input side to the output side and for modulating the input electrical power before it is provided to the output terminals. The switches employed in such switching arrangements are typically comprised of solid state switches, such as MOSFET transistors for example. The transformer provides for galvanic isolation between the input and output and voltage step-up or step-down.
Often in switch mode DC-DC converters, the switches are activated by means of a control circuit controlling the phase angle, frequency (i.e., frequency modulation) and/or duty cycle of the switches in the switching arrangements to assume an ON-state (switch closed) or an OFF-state (switch open) in order to regulate voltage and current. For example, in the case of MOSFET switches, the control circuit is adapted to provide a gate voltage to switch the source-drain conduction channel ON (conducting) or OFF (non-conducting) in a timed manner. A rectifying diode may also be implemented by a three-terminal device, such as a MOSFET, by operating the control circuit driving the three-terminal device in a synchronous rectification mode.
In order to implement DC-DC converters used in high power applications, cascaded DC-DC converters with multi-level scheme may be used.
While existing pulse width modulation, phase shift control or frequency modulation control techniques that are employed for controlling switching in a DC-DC converter arc effective for purposes of regulating voltage and current to a desired level, such control techniques can suffer from efficiency issues when they are used over a wide regulation range. To alleviate this, cascaded DC-DC converters have been used in prior art with coarse regulation performed by bypassing or engaging selected number of converters with external switches, and fine regulation done through modulation changes in a narrow range. The external switches result in increased size and cost, and also lead to additional power losses due to conduction drops.
Therefore, it is desirable to provide DC-DC converters and associated control scheme for operating the converters that increases the efficiency of the converters. It is also desirable that the DC-DC converters include a minimal number of switches therein necessary to regulate voltage and current, such that the number of switches is reduced and a compact DC-DC conversion system is provided.